This invention relates generally to machine tools and methods and means for attaining machining precision therein and more particularly to a system for compensatively correcting for thermal displacement which corrects machining errors due to thermal deformation in machine tools, and which can be utilized in machine tools of a general type.
Among the various kinds of machine tools, there are several of the type, such as a milling machine or a machining center, which is provided with a main spindle head having a main spindle on which a tool is to be mounted, a column on which the main spindle head is supported in a manner permitting it to ascend and descend, and a table on which a workpiece to be machined by the tool is mounted and held. In general, in a machine tool of this type, the principal cause of machining error is the relative displacement due to variations in the relative positional relationship between the main spindle holding the tool and the table.
This relative displacement occurs because of the following circumstances. In the essential structural members and mechanisms intervening between the main spindle and the table, such as the main spindle head, the column, a bed, a saddle, the table, and ball screws, the temperature distribution is generally not uniform and gives rise to uneven thermal expansions of these various parts, which thereby undergo mutually different elongation or contraction, warping, and deformation, resulting in the relative displacement, which is further increased by the compounding of these irregular deformations.
From the past, in order to prevent machining errors due to such causes, various measures such as measure with respect to the heat source itself which is the cause of thermal deformation and measures for compensation and correction according to measurements of the thermal deformations have been devised and practiced.
In one example of the former measure, lubrication oil which has been temperature controlled is circulated through heat generating parts or parts whose temperatures are expected to rise thereby to absorb heat and suppress temperature rise. In another example of the former measure, the machine is so assembled that its precision will be maintained at the temperature of its machining operation, and, prior to entering into an actual machining operation, the machine is put through a warm-up run. While methods of this character may be effective as measures for suppressing thermal displacement within certain ranges, they are not basic solutions of the problem of thermal displacement. More specifically, as the machines progressively become larger and more complex, even slight temperature variations have a great influence on their machining precision, and it is impossible to maintain the machining precision at a constant level of an order which is unaffected by the temperature or temperature gradient of the machine. Furthermore, an attempt to reduce the temperature variation as much as possible will give rise to the necessity of using expensive equipment such as an accurate temperature control system and a large-capacity heat exchanger.
An example of the latter of the above mentioned measures is a method which comprises detecting the quantity of thermal deformation of the column by means of an electric level and correcting the error component of the position of the main spindle according to the quantity thus detected. However, an electric level has as its reference the gravitational direction, which is a reference outside of the machine body. For this reason, a plurality of levels are necessary, and the operational processing becomes complicated. Further, depending on the position of the main spindle, it is necessary to convert the correction amount each time. Another example of the latter measure comprises providing a reference plate within the column, detecting any deviation between the reference plate and the column by means of a differential transformer, and correcting the position of the main spindle. By this method, however, since the reference plate is provided within the column, the construction becomes complicated. Moreover, since the reference plate and the detecting element are continually in contact and undergoing mutual sliding movement, there is a possibility of development of errors in the detected values due to frictional abrasion of the detecting element during machining.
In addition, as an improvement of the latter measure, there is a method which, as disclosed in Japanese Patent Publication No. 8177/1985, comprises detecting displacement quantity by means of an optical device, operating a compensation device of improved precision and simplified construction in response to the detection signals thus obtained, and compulsorily correcting the displacement quantity by means of a heating device.
This is a thermal displacement correction system for a machine tool which, for correcting displacement in the column arising from inclination of the column, comprises a light source of the optical device and a reflecting mirror disposed apart from and facing this light source, both being disposed at specific positions on the column centerline, a light point displacement detector which is disposed on a column reference surface perpendicular to the column centerline at the base part of the column, and which detects displacement of a beam of light emitted by the light source and reflected by the mirror and generates a displacement detection signal, a computing device for receiving this detection signal as input and computing the displacement of the column from the reference surface to the main spindle to produce an output signal, and a correction circuit for operating in response to this output signal and a signal from a numerical control system to shift and adjust the position of the table and to correct the position of the main spindle.
In this case, the light from the light source is in the form of a light beam, and the displacement of the mirror is detected by the light point displacement detector. The corrective results of this method are sensitively influenced by shimmering or flickering due to air, and problems arise in measurement accuracy. Furthermore, when thermal displacement of the structure on which the light source is mounted occurs with variation of the temperature of the surroundings, minute angular variations of the light source are directly magnified and become a source of measurement errors. A fundamental inadequacy of this method is that, when correction is carried out by determining displacement due to deformation of only the column, it is merely correcting one part of the essential structure which gives rise to machining error and does not constitute a sufficiently satisfactory measure for eliminating machining errors.